Formosus was born around 816, probably in Rome. He became Cardinal-Bishop of Porto in 864. He undertook diplomatic missions to Bulgaria (866) and France (869 and 872), and he persuaded Charles the Bald, King of France, to be crowned by the pope.

As early as 872 he had been a candidate for the papal see. But due to political complications he left Rome and the court of Pope John VIII in that year. John convened a synod, and Formosus was ordered to return or be excommunicated on charges that he had aspired to the Archbishopric of Bulgaria and the Chair of Peter, had opposed the emperor and had deserted his diocese without papal permission, had despoiled the cloisters in Rome, had performed the divine service in spite of the interdict, and had conspired with certain iniquitous men and women for the destruction of the papal see. The condemnation of Formosus and others was announced in July 872. In 878 the sentence of excommunication was withdrawn, after he had promised never to return to Rome or exercise his priestly functions.

John's successor Pope Marinus I in 883 restored him to his Diocese of Porto. Following the reigns of Marinus, Pope Hadrian III (884-885) and Pope Stephen V (885-891), Formosus was elected Pope on October 6 891.

Formosus was forced to crown Duke Guido of Spoleto Roman Emperor in April 892. Other immediate issues were that in Constantinople, the patriarch Photius had been ejected and Stephen, the son of Emperor Basil I, had taken the office. There was a quarrel between the Archbishops of Cologne and Hamburg concerning the Bishopric of Bremen. In the contest between Odo, Count of Paris and Charles the Simple for the French crown, the pope sided with Charles.

Formosus persuaded Arnulf of Carinthia to advance to Rome and liberate Italy. In 894, Arnulf subjugated all the country north of the Po. Guido died in December leaving his son Lambert in the care of his mother Agiltrude , an opponent of the Carolingians. In the autumn of 895, Arnulf undertook his second Italian campaign, and in 896 he was crowned by the pope in Rome. The new emperor moved against Spoleto but was struck with paralysis on the way and was unable to continue the campaign.

In April 896, Formosus died. He was succeeded by Pope Boniface VI.

Pope Stephen VII, the successor of Boniface, influenced by Lambert and Agiltrude sat in judgment on Formosus in 897, in what was called the Cadaver Synod. The corpse was disinterred, clad in papal vestments and seated on a throne to face all the charges from John VIII. The verdict was that the deceased had been unworthy of the pontificate. All his measures and acts were annulled, and the orders conferred by him were declared invalid. The papal vestments were torn from his body, the three fingers from his right hand which the pope had used in consecrations were cut off and the corpse was then thrown into the Tiber. Following the death of Stephen the body was reinterred in St. Peter's. Further trials of this nature against deceased persons were banned. But Pope Sergius III (904-911) reapproved the decisions against Formosus. Sergius demanded the re-ordination of the bishops consecrated by Formosus, who in turn had meanwhile conferred orders on many other clerics, causing great confusion. Later the validity of Formosus's work was re-reinstated. The decision of Sergius has been disregarded by the church as Sergius was an immoral man guilty of having murdered others.